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Pistons are elements that lay an important role in engine performance and reliability. They in fact act as valves on the gas produced by mixture burning in the combustion chamber. These gases at high temperature and pressure have a very high quantity of energy that can be transformed to mechanical energy through the con-rod and crank-shaft system. This system is moved thanks to the capacity of the piston in sealing the volumes over and under it in the cylinder and crank-case. Also in this passage mechanical energy of the piston expressed through a longitudinal movement is transformed in rotational mechanical energy. All this happens with great forces acting on the piston (mechanical stress) that are added to the high temperatures that the piston reaches in contact with the burned gases (thermal stress).
Pistons move longitudinally in the cylinder, but their speed changes constantly in a sinusoidal way. This means the piston starting for example at top dead corner (TDC) has a speed equal to zero. Then it accelerates quickly reaching top speed when the con-rod is at 90° respect to the crank-shaft arm. After that speed decreases again and is null at bottom dead corner (BDC). This quickly changing speed indicates strong accelerations (both negative, deceleration, and positive). Any acceleration generates a force called inertial force. Inertia is the capacity of every body to oppose itself to acceleration (negative and positive). The opposition is the inertial force. Inertia increases with weight of the element and the square value of otational speed of the crank-shaft. Maximum negative and positive inertial forces are at TDC and BDC.
In addition to the longitudinal movement along the cylinder axis he piston has orthogonal movements that are determined by the lateral forces generated by the inclination of the con-rod respect to the cylinder longitudinal axis. Since the diameter of the piston is slightly smaller than the one of the cylinder, the piston can move sideways inside the cylinder. When the piston is moving upwards it is pressing on one side of the cylinder liner. When it passes the TDC and comes down the piston presses on the opposite side of the cylinder liner. This is due to the fact that the con-rod is inclined on the opposite side.
Total forces on the piston
In addition to the inertial forces acting on the piston, the head of the piston has to deal with another mechanical stress, such as the high pressure coming from the burned gases in the combustion chamber, and a thermal stress as the temperatures transmitted from these same gases. In particular in two stroke engines a cycle is completed in 360° of the crank-shaft, which means the piston has less time to cool down respect to the situation of a four stroke engine. In two-stroke competition engines such as kart engines the highest temperatures are reached at the centre of the piston head and can reach 400°C (752°F) in air cooled engines and 360°C (680°F) in water cooled engines.
Heat transmitted from the burned gases in the combustion and expansion phases to the piston head is then given to the cylinder through the ring(s). From 30% to 60% of the total heat energy is transmitted this way and helps cooling the piston. Piston temperature increases around 3°C every 100 revs/min and also 15° every bar of increases of medium pressure in the combustion chamber. Also combustion timing and compression ratio influence the temperature of the piston.
All these factors stress strongly all the structure of the piston and this is why materials and shape are well designed to obtain a very light and strong/resistant component. Aluminium for control and limitation of deformation given by high temperatures and additional elements added to the piston are key for obtaining a good result. Shapes also have changed a lot during the years, but finally they seem to be today very similar to one another especially in two-stroke competition engines for karts.
We will see in the next issue exactly how pistons are built and how the selection of the materials is made of a series of elements that help make a resistant component with also good thermal characteristics that help reduce friction, increase performance, reliability and durability of the piston and the entire engine.